Joint Retrieval of Co and Vibrational Temperature from Mipas-envisat

MIPAS is a limb viewing fourier transform spectrometer operating in the infra-red. It scans the tangent altitude range 6-68 km at a vertical resolution of approximately 3km. CO has various natural and anthropogenic sources including forest fires and industry. In the troposphere, CO is the main sink of the principal oxidising agent the hydroxyl radical and so reduces its capacity for the removal of other atmospheric pollutants. In the stratosphere, its intermediate lifetime makes it useful as a tracer for stratospheric motions and processes of tropospheric-stratospheric exchange. However, the retrieval of CO from a limb sounding IR instrument is complicated because of non-LTE (non-local thermodynamic equilibrium) effects. Ordinarily, the internal vibrational energy level populations are controlled by collisions between molecules and follow the translational (kinetic) energy distribution of the ambient gas (LTE). However, if collisions are infrequent, radiative processes can lead to a non-Boltzmann distribution of the internal energy level populations (non-LTE). CO is strongly affected by non-LTE in the IR down to around 40km and modelling these processes is difficult. These effects are especially problematic in the limb viewing geometry due to the long path length viewed through the upper atmosphere. However, using a joint CO, vibrational temperature (Tv) retrieval it is possible to estimate the non-LTE effects without modelling the energy level populations. Instead, we retrieve a parameterisation of the non-LTE effects, Tv, directly from the emission spectra.